AutoMath
The car itself

Tire Size Calculator

Convert tire-size notation into real dimensions and see exactly how a different size changes overall diameter, your true speed, and revolutions per mile.

Your numbersSaved on this device only
Original — 225/45R17
New — 245/40R18
Overall diameter change

+2.98%

25" → 25.7"

✅ Within the ±3% fitment guideline
At an indicated 60 mph you're actually doing 61.8 mph — a minor, generally acceptable deviation.
True speed
61.8 mphat 60 mph shown
Speedo error
+2.98%
Revs/mi — original
807.6
Revs/mi — new
784.2

What this computes

A tire size is a compact spec, not a measurement. This unpacks it: sidewall height, overall diameter in mm and inches, circumference, and revolutions per mile — then compares two sizes for diameter change, speedometer error, and the true speed behind a given indicated reading.

The math

sidewall   = width × aspect / 100        (mm)
diameter   = rim × 25.4 + 2 × sidewall   (mm)
revs/mile  = 1,609,344 / (π × diameter)
true speed = indicated × (newØ / origØ)

The speedometer is calibrated to the original tire, so the speed error is exactly the overall-diameter change percentage — bigger tire, you're going faster than it shows.

Plus-sizing done right changes the wheel and the look while leaving overall diameter — and your speedometer — almost untouched.

How to use this

  1. Enter the size off the sidewall exactly — width, aspect, rim — for both the original and the size you're considering.
  2. Watch the diameter percentage, not just the looks. Keeping it near 0% preserves calibration.
  3. Set your usual cruising speed to see the real number behind the dial after the swap.
  4. Use it to plus-size: raise the rim, lower the aspect, and tune the width until the diameter change is near zero.

The ±3% rule

Staying within ±3% of original overall diameter is the practical safe window because beyond it:

  • Speedometer/odometer drift becomes legally and financially meaningful (mileage, tickets, warranty).
  • ABS, traction, and stability control rely on wheel-speed assumptions tied to rolling radius.
  • Clearance against suspension, fenders, and brakes gets risky as diameter grows.

What this calculator doesn't model

  • Manufacturing tolerance. Real tires deviate slightly from nominal size by brand.
  • Load deflection. The loaded rolling radius is a touch smaller than the free diameter.
  • Load index / speed rating. Geometry only — always match or exceed the original ratings.
  • Wheel width and offset fitment, which also constrain a tire change.

Frequently asked questions

How do you read a tire size like 225/45R17? +
225 is the section width in millimetres. 45 is the aspect ratio — the sidewall height as a percentage of the width (so 225 × 0.45 ≈ 101 mm of sidewall). R means radial construction. 17 is the wheel (rim) diameter in inches. Overall tire diameter is the rim diameter plus two sidewalls.
Why does changing tire size affect the speedometer? +
The speedometer counts wheel rotations and assumes the original tire's circumference. A larger-diameter tire travels farther per rotation, so you're actually going faster than the speedometer shows; a smaller tire is the reverse. The error percentage equals the overall-diameter change percentage.
What is plus-sizing? +
Fitting a larger-diameter wheel with a lower-profile tire so overall diameter stays roughly the same — e.g. going from 225/45R17 to 245/40R18. Done correctly the diameter barely changes, preserving speedometer accuracy and clearance while changing looks and handling. This calculator shows whether your intended plus-size actually keeps the diameter close.
How much diameter change is acceptable? +
The widely-used guideline is to stay within ±3% of the original overall diameter. Beyond that, speedometer/odometer error grows, ABS and traction/stability calibration can be affected, gearing and fuel-economy readings shift, and clearance problems become likely. The calculator flags whether your swap is inside or outside this range.
Does a bigger tire help or hurt? +
It depends on the goal. A taller tire raises ride height and effective gearing (lower RPM at speed, sometimes worse acceleration) and makes the speedo under-read. A shorter/wider tire can sharpen handling but stiffen the ride. There's no universally 'better' — match the change to your purpose and keep diameter within range unless you also recalibrate.
Is this exact? +
The geometry is exact for the stated dimensions, but real tires vary slightly from their nominal size by brand and model, and load deflection changes the rolling radius a little. AutoMath is an educational tool — treat the output as accurate to within those real-world tolerances.

Related calculators

What changes and what breaks: plus-sizing tires and your speedometer.

AutoMath is an educational tool. The numbers above depend entirely on the dimensions you provide and are not a fitment guarantee.